1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly, to an apparatus and method for transmitting and receiving a Forward Shared Control Channel (F-SCCH) in a mobile communication system supporting a multi-antenna technology.
2. Description of the Related Art
Mobile communication systems are developing into high-speed, high-quality wireless packet data communication systems in order to provide data services and multimedia services beyond voice-oriented services.
Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA) proposed by 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) proposed by 3GPP2, and 802.16 proposed by IEEE, are being developed to support high-speed, high-quality wireless packet data transmission services.
The 3rd generation wireless packet data communication systems, such as the above-stated HSDPA, HSUPA and HRPD, use such technologies as an Adaptive Modulation and Coding (AMC) method and a Channel-Sensitive Scheduling method to improve transmission efficiency.
With the use of the AMC method, a transmission apparatus can adjust the amount of transmission data according to the channel state. That is, in a poor channel state, the transmission apparatus reduces the amount of transmission data to decrease a reception error probability to a desired level. In a good channel state, the transmission apparatus increases the amount of transmission data to increase the reception error probability to a desired level, thereby ensuring efficient information transmission.
In addition, with the use of the Channel-Sensitive Scheduling resource management method, the transmission apparatus selectively services a user having a superior channel state among several users, thereby contributing to an increase in the system capacity, compared to the case where the transmission apparatus allocates a channel to one user and services the corresponding user. Such an increase in system capacity is called ‘multi-user diversity gain’.
In sum, the ACM method and the Channel-Sensitive Scheduling method are methods in which the transmission apparatus applies a suitable modulation and coding technique at the most efficient time determined based on partial channel state information fed back from a reception apparatus.
To implement the ACM method and the Channel-Sensitive Scheduling method, the reception apparatus should feed back channel state information to the transmission apparatus. The channel state information fed back by the reception apparatus is called a Channel Quality Indicator (CQI).
Recently, intensive research has been conducted to replace Code Division Multiple Access (CDMA), the multiple access scheme used in the 2nd and 3rd mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation system.
In this context, 3GPP and 3GPP2 are presently discussing the standardization for the evolved system using OFDMA. The evolved system using OFDMA, compared to the system employing CDMA, can expect an increase in the system capacity.
One of several reasons that the OFDMA scheme results in a capacity increase is that the OFDMA scheme can perform scheduling in the frequency domain (hereinafter referred to as ‘Frequency Domain Scheduling’). That is, although capacity gain was obtained through the Channel-Sensitive Scheduling method using the characteristic that a channel varies with the passage of time, higher capacity gain can be obtained with the use of the characteristic that a channel varies according to the frequency.
However, in order to support Frequency Domain Scheduling, the transmission apparatus should previously acquire channel state information separately for each frequency. In this case, since there is a need for CQI feedback separately for each frequency, the reception apparatus and the transmission apparatus may suffer from an increase in the signaling load due to transmission/reception of the CQI feedback.
In the next generation system, studies have been made regarding the introduction of a Multiple Input Multiple Output (MIMO) technology using multiple transmit/receive antennas. The MIMO technology is a technology that simultaneously transmits multiple data streams over the same resources using multiple transmit/receive antennas. It is known that the MIMO technology is a method that can increase transmission throughput at the same error probability by transmitting multiple low-modulation order data streams rather than increasing a modulation order in the good channel state. In the MIMO technique, a dimension where an individual data stream is transmitted is called a layer, and a method of separately applying AMC according to the channel state of each layer can contribute to an increase in the entire system capacity.
For example, Per Antenna Rate Control (PARC) is a technology in which every transmit antenna transmits a different data stream, and in this technology, the layer is each transmit antenna. In this case, each of the multiple transmit antennas may experience a different channel, and the PARC technique applies AMC so as to transmit more data via a transmit antenna(s) having a good channel state, and reduces the amount of data transmitted via a transmit antenna(s) having a poor channel state.
As another example, there is Per Common Basis Rate Control (PCBRC), and in the PCBRC technology, the layer is a fixed transmission beam. Therefore, the PCBRC technique transmits more data over a transmission beam(s) having a good channel state, and reduces the amount of data transmitted over a transmission beam(s) having a poor channel state.
When MIMO is implemented using multiple antennas, a precoding method is used to adaptively form transmission beams according to the channel state. The term ‘precoding’ as used herein refers to an operation in which the transmission apparatus previously distorts a transmission signal in the step before it transmits the signal via a transmit antenna. When precoding is implemented by linear combining, the precoding process can be expressed by Equation (1).x=Es  (1)
In Equation (1), ‘s’ is a K×1 vector and denotes a desired transmission signal, and ‘x’ is an M×1 vector and denotes an actual transmission signal. Further, ‘K’ denotes the number of symbols simultaneously transmitted over the same resources by MIMO, and ‘M’ denotes the number of transmit antennas. In addition, ‘E’ is an N×K matrix, and denotes precoding. That is, Equation (1) expresses a preceding scheme E applied when a MIMO transmission apparatus with M transmit antennas simultaneously transmits K signal streams.
A precoding matrix E is adaptively determined according to a MIMO transmission channel. However, the transmission apparatus, when it has no information on the MIMO transmission channel, performs precoding according to the feedback information reported by the reception apparatus. To this end, a precoding codebook including a finite number of precoding matrixes E should be previously set between a transmitter and a receiver, and stored therein. Therefore, the reception apparatus should select a precoding matrix E most preferred in the current channel state from the previously stored precoding codebook taking the current channel state into account, and feed back information on the selected precoding matrix E to the transmission apparatus. Then the transmission apparatus performs MIMO transmission by applying precoding based on the received feedback information for the precoding matrix E.
Regarding the transmission signal of Equation (1), a signal received at the reception apparatus after experiencing a MIMO channel H is defined by Equation (2).y=Hx+z=HEs+z  (2)
In Equation (2), ‘y’ and ‘z’ are both an N×1 vector, and denote a signal and a noise received at N receive antennas, respectively, and ‘H’ is an N×M matrix, and denotes a MIMO channel. The received signal undergoes a reception combining process so that a Signal-to-Interference and Noise Ratio (SINR) for a transmission signal stream of each layer may be improved. A signal r, after undergoing the reception combining process, is defined by Equation (3).r=Wy=WHx+Wz=WHEs+Wz  (3)
In Equation (3), ‘W’ is an N×N matrix and denotes a reception combining process, and ‘r’ is an N×1 signal vector. In order to better receive a transmission signal stream of each layer, a reception technique such as interference cancellation and/or Maximum Likelihood (ML) reception can also be used.
A Single-Code Word (SCW) scheme and a Multi-Code Word (MCW) scheme are distinguishable according to the number of coded packets from which multiple signal streams transmitted by the MIMO technique are generated.
In the SCW scheme, one codeword is transmitted over multiple layers made by the MIMO technique regardless of the number of layers, and the MCW scheme transmits one different codeword over each of multiple layers made by the MIMO technique.
The MCW scheme is advantageous in that a receiving side can obtain additional gain by way of a reception process such as interference cancellation. This is because the reception apparatus can determine the success/failure in decoding of each codeword using a Cyclic Redundancy Check (CRC) applied to each codeword. However, the MCW scheme, as it increases the number of transmission codewords, wastes additional resources that it linearly increases to apply CRC, and also increases the complexity of the reception apparatus.
A Dual CodeWord (DCW) scheme is compromised to obtain a rate improvement effect of the MCW scheme while compensating for the above drawbacks. In the DCW scheme, a maximum of two codewords are transmitted over multiple layers made by the MIMO technique regardless of the number of layers.
FIG. 1 illustrates an exemplary structure of an SCW MIMO transmission/reception apparatus to which the present invention is applied.
Referring to FIG. 1, a desired transmission data stream is converted into one coded packet signal stream after undergoing a channel coding and modulation process 101. For MIMO transmission, the signal stream is demultiplexed at a demultiplexer 103 into K signal streams. The K demultiplexed signal streams are reshaped into M signal streams to be transmitted via their associated transmit antennas by means of a precoder 105. This process is provided so that K signal streams are transmitted over different transmission beams.
The M precoded signal streams are transmitted via transmit antennas 109a-109m by way of transmission processors 107a-170m, respectively. The transmission processors 107a-170m include not only the process of generating CDMA and/or OFDMA signals and but also the filtering and/or Radio Frequency (RF) processing process performed in their associated antennas.
The transmitted signals are received at N receive antennas 111a-111n, and the signals received via the receive antennas are restored into baseband signals by means of reception processors 113a-113n, respectively. After the reception-processed signals are converted into K signal streams by means of a reception combiner 115, the K signal streams are restored into one desired transmission signal stream after undergoing multiplexing in a multiplexer 117. Finally, the restored signal stream is restored into a desired transmission data stream by means of a demodulation and channel decoding unit 119.
As described above, in the SCW MIMO scheme, because the transmission apparatus generates multiple transmission signal streams using one channel coding and modulation process 101, it only needs to receive one CQI feedback. However, the number of MIMO transmission signal streams, i.e., the number K of MIMO layers over which the signal streams are transmitted, should be adjusted according to the channel state. The number K of MIMO transmission layers over which signal streams are transmitted is referred to herein as a ‘Rank’. Therefore, the SCW MIMO feedback information is composed of one CQI representative of a channel state of a MIMO transmission layer, and the number Rank of transmission layers, required according to the channel state.
FIG. 2 illustrates an exemplary structure of a DCW MIMO transmission/reception apparatus to which the present invention is applied.
Referring to FIG. 2, in the DCW MIMO scheme, unlike in the SCW MIMO scheme, two different coded packet signal streams are transmitted over a MIMO layer.
A desired transmission data stream is demultiplexed at a demultiplexer 201 into two data streams, and the demultiplexed data streams are converted into modulation signal streams after undergoing different channel coding and modulation processes 201-1 and 201-2, respectively. The succeeding transmission process is the same as that of the SCW MIMO scheme, and the modulation signal streams are converted into signals to be transmitted via M transmit antennas 209a-209m, after undergoing a precoding process at a precoder 205 and transmission processing at transmission processors 207a-207m for their associated transmit antennas.
A DCW MIMO reception process is also the same as the SCW MIMO reception process in several steps immediately after signal reception. In particular, although the reception apparatus of FIG. 2 uses an interference canceller 21-, by way of example, it can use a reception method of another type.
Signal received via N receive antennas 211a-211n are restored into transmission signals associated with corresponding layers after undergoing reception processors 213a-213n, and a reception combiner 215 in sequence. The restored signals include interference to/from each other. In DCW MIMO, because the transmission signals underwent different coding and modulation separately for each layer, a receiver can remove the first restored signal of a particular layer to cancel the interference that the signal exerts on another layer.
The use of the interference canceller 219 can improve channel capacities of MIMO layers, making it possible to transmit more data through DCW MIMO transmission. An interference cancellation-based reception process will be described below. When one modulation signal stream is successfully restored through demodulation and channel decoding 217, the reception process cancels interference using the restored signal at interference canceller 219. The interference-canceled signal stream 223 is delivered back to the demodulation and channel decoding unit 217 where based thereon, it restores a second modulation signal stream. Finally, the two restored data streams are restored into one desired transmission data stream after undergoing multiplexing at multiplexer 221.
The MCW MIMO transmission/reception apparatus is not so different from the DCW MIMO transmission/reception apparatus in structure. The transmission apparatus supporting MCW MIMO transmits a different codeword separately for each MIMO layer made through precoding, and the reception apparatus supporting MCW MIMO cancels the interference contributed by the first restored signal stream in order of the first restored signal stream while performing demodulation and channel decoding separately for each layer, and repeatedly performs interference cancellation, and demodulation and channel decoding thereon until the interference is fully canceled. Since an operation of MCW MIMO can be simply analogized by extending the operation of DCW MIMO, a detailed description thereof will be omitted herein for simplicity.
A downlink (DL) control channel will be described below. A DL control channel is a channel including the control information that a terminal (or User Equipment (UE)) needs to restore a signal transmitted from a base station. Generally, the downlink control channel includes the following information.
1. User Equipment Identification (UE ID): UE ID is information based on which a terminal determines the presence of a signal being transmitted to the terminal itself. Since a CRC based on a particular UE ID is generally inserted into DL control information, if the terminal has successfully restored the DL control information, it recognizes the corresponding control information as information for the corresponding terminal.
2. Down Link Resource Block (DL RB) allocation information: If the terminal has successfully restored the DL control information, it determines based on the DL RB information over which resource block its actual data is transmitted.
3. Transport Format (TF): TF indicates a modulation and coding scheme of transmission signal. A terminal, if it uses AMC, should have TF information in order to perform a demodulation and channel decoding process.
4. Hybrid Automatic Repeat Request (HARQ) related information: HARQ is an operation in which a receiver transmits, to a transmitter, information indicating the success/failure in reception of a transmission packet. The transmitter transmits another packet when the receiver has succeeded in the packet reception, and the transmitter retransmits the previous packet when the receiver has failed in the packet reception. The term ‘HARQ-related information’ as used herein refers to information related to HARQ indicating whether a transmission signal is an initial transmission signal or a retransmission signal. Based on this, the terminal determines whether it will combine the received packet with the previously received packet and perform decoding thereon, or it will newly perform a decoding operation.
In MIMO transmission, aside from the 4 types of information stated above, additional information can be transmitted over a DL control channel. For example, when precoding is applied, the additional information can be precoding information because there is a need to provide the terminal with information indicating which precoding scheme is applied.
Therefore, in SCW MIMO transmission to which precoding is applied, the necessary DL control channel information should include not only the UE ID, DL RB, TF and HARQ-related information, but also the precoding information.
In DCW MIMO transmission to which precoding is applied, because the number of transmission codewords is 2, the TF and HARQ-related information corresponding to each codeword should be transmitted. That is, the DL control channel information necessary for DCW MIMO transmission to which precoding is applied, includes UE ID, DL RB, TF #1 (TF of a first codeword), TF #2 (TF of a second codeword), HARQ-related information #1 (HARQ-related information of the first codeword), HARQ-related information #2 (HARQ-related information of the second codeword), and precoding information.
Herein, the precoding information, which is information indicating which precoding matrix is applied, includes codeword mapping information between the layer configured through precoding, and the transmission codeword.
A description will now be made of a method for configuring a DL control channel for MIMO transmission to which preceding is applied.
FIG. 3 illustrates DL control channel information for SCW MIMO, to which the present invention is applied.
Referring to FIG. 3, it can be appreciated that the DL control channel information includes UE ID 301, DL RB 303, precoding information 305, TF 307, and HARQ-related information 309, wherein the order of the information is meaningless. If predefined synchronized HARQ is applied for a retransmission time and resources required for retransmission, the HARQ-related information 309 can be omitted.
FIG. 4 illustrates DL control channel information for DCW MIMO, to which the present invention is applied.
Referring to FIG. 4, it can be appreciated that the DL control channel information includes UE ID 401, DL RB 403, preceding information 405, TF #1 407-1, HARQ-related information #1 409-1, TF #2 407-2, and HARQ-related information #2 409-2. Similarly, the order of the information is meaningless, and if synchronized HARQ is applied, the HARQ-related information 409-1 and 409-2 can be omitted.
When the TFs and HARQ-related information, the number of which corresponds to the number of codewords, are acquired as shown in FIG. 4, the general DL control channel information for MCW MIMO can be configured.
Even in the situation where an agreement to perform DCW MIMO transmission is made between a transmitter and a receiver, if the number of activated layers is 1, only one codeword is transmitted. A technique of adaptively adjusting the number of activated layers according to the state of a MIMO channel in this way is called ‘Rank Adaptation technique’. When an SINR is low or a correlation between channels is high, even though it is possible to configure M layers, the number of layers over which signal streams are actually transmitted should be set lower than M. Herein, the layer over which signal streams are actually transmitted is referred to as an ‘activated layer’, and the number of activated layers is referred to as a ‘transmission layer’ or ‘Rank’.
When a transmission Rank (hereinafter ‘Rank’ for short) is greater than or equal to 2 (Rank≧2), the DCW MIMO transmission scheme transmits 2 codewords without condition. However, when Rank is 1 (Rank=1), the DCW MIMO transmission scheme cannot transmit only 1 codeword. In this way, for Rank=1, SCW MIMO and DCW MIMO are equal to each other in operation, and when precoding is applied, it can be considered as an Adaptive Beamforming scheme.
In the DL control channel information for DCW MIMO shown in FIG. 4, the TF #2 407-2 and the HARQ-related information #2 409-2 are information unnecessary for Rank=1. However, the conventional mobile communication system has a channel structure that transmits the unnecessary information, i.e., TF #2 407-2 and HARQ-related information #2 409-2, even for Rank−1 transmission (i.e., Rank=1 transmission) of DCW MIMO.
Therefore, there is a need to define a structure of a downlink control channel for DCW MIMO in a MIMO system such that the structure should adapt to a change in Rank.